1. Field of Invention
The present invention generally relates to microwave motion sensors and more particularly a plurality of microwave radiating elements including a reflector for enhanced lookdown ability.
2. Description of Prior Art
Increasingly, security systems and automated manufacturing systems, as well as other automated processes are employing microwave (MW) motion sensors for motion detection. MW motion sensors typically include a microwave transceiver which is an active device employing electromagnetic waves lower in frequency than visible light. A MW motion sensor transmits a microwave signal toward a region to be monitored and in the event that movement is detected within the region, the microwave signal is reflected back (echo) from such movement and is modulated due to the Doppler Effect. When a signal is reflected from a moving object (target) it is shifted in frequency. This shift in frequency is called the Doppler Effect and is directly proportional to the targets velocity relative to the sensor. The maximum frequency shift is when the target is moving straight at the sensor and minimum frequency shift is observed if the target is moving at 90 degrees to the target. All Doppler sensors use this principle.
In certain security and other automated applications or systems, a MW sensor may be mounted on a wall to monitor a room or part of a room. In such a situation a microwave reflector is employed to enhance the coverage of the MW motion sensor. For example, a MW transmitter and receiver employing a reflector is described in PCT WO 97/43662.
A front and side view of a prior art MW motion sensor 100 is shown in FIGS. 1A and B. The prior art MW motion sensor includes an antenna 110 having a plurality of microstrip radiating elements (antenna patches) 120 and a strip of copper foil 130 disposed in front of the antenna aperture so as to reflect an upper portion of the emitted radiation from the plurality of microstrip radiating elements 120. FIGS. 1A and 1B depict the antenna aperture of the prior art MW motion sensor 100 as it is well known in the art that an antenna aperture is the portion of a plane surface very near the antenna and normal to the direction of maximum radiant intensity, through which the major part of the radiation passes. In other words, the prior art reflector or strip of copper foil 130 provides enhanced microwave amplitude at low incident angles, thereby effectively reducing a blind zone beneath the microwave transmitters by reflecting part of the main beam.
However, the aforementioned prior art MW motion sensor provides enhanced detection underneath the sensor, but provides this enhanced signal by reflecting part of the main beam. This may be detrimental to detection in the protected region by the main beam from the antenna.